Talk:Broadcast television systems

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Some thoughts for future research:

• There should be a history section describing how the formats evolved and when various systems were phased out in the countries that used them.
• Whatever happened to system C? I know I've seen a version of this table with the parameters, but I can't find it any more.
• Why did the French go with 819 lines? Why did the French stick with AM audio and positive video modulation (thereby unnecessarily bringing system L into existence) when nobody else did (including their SECAM allies in the OIRT, and even their own overseas departments, which implemented system K instead)?
  • Because they were French :P Seriously, it was just the way that television in France was developed. Remember that at the time, television was a competitive project, and each country was trying to develop the system, perhaps in the hope that their system would gain acceptance, and could be exported, along with patent rights, etc. In England, we arrived at 405-lines, because that was considered to be a 'broadband' signal at the time. Plus it was a case of being able to physically transmit a picture on the available wavebands, thus the picture-standard had to be developed to be compatible with that. (RM21 00:39, 26 June 2006 (UTC))
• Should there be a section on bandplans? They seem to be few in number, and they certainly had an influence on the technical parameters of the systems (particularly on VHF, as witness system B's 7-MHz channels, and system L's -6.5 MHz audio offset).
• What's up with Brazil?
• Can the layout of the big table of systems be improved any?

Just added a section on the French 819 line standards. Why they opted for variations in their 625 line standard is not quite clear, maybe they just wanted to be different. Just like the UK :). Anorak2

Hey, the UK only went for System I because we (Ireland) were using it - couldn't deny those Welsh people their RTE One :p Seriously, System I was used in Ireland first by around two years but I'd assume the BBC had a huge part in developing it. --Kiand 14:41, 11 March 2006 (UTC)

Yes, but in Ireland, the difference was the previous systems were upgraded from 405 to 625-lines, on vhf. In the UK, vhf channels were seen as troublesome [because of problems with temperature inversions and long-distance reception], and old-fashioned, so they were closed. The UK could have as easily opted for the B/G system, which would have had the advantage of being compatible with Europe; this would almost certainly have been chosen today. (RM21 00:39, 26 June 2006 (UTC))

Concerning Brazil: They are regular system M just like North America, but opted for PAL colour probably following an advertising campaign by Telefunken. At least that is what Walter Bruch (PAL inventor) writes in one of his essays. Is it worth mentioning in the main article? Anorak2 11:04, 1 February 2006 (UTC)

If that's the extent of the story it's probably not worth repeating. I still hold out hope that there is a specific government report or published history which someone can cite for an official explanation. 121a0012 05:52, 2 February 2006 (UTC)

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Contents 1 System J 2 jidder, etc. 3 System I 4 Channels 5 Frames 6 Modulation 7 Audio

[edit] System J

Why is there no system J?? 66.32.240.129 14:07, 6 Sep 2004 (UTC)

Because I and J are often confused. Anorak2 11:04, 1 February 2006 (UTC)

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"(However, the choice of a 25 Hz frame rate has some unfortunate consequences for the telecasting of movies, most of which are filmed at a 24 Hz frame rate.)"

As I see it, the only unfortunate consequence for movies is that they run slightly faster and unless the soundtracks are digitally pitch-corrected they're about a semitone highter. But since the movies are screened at the same frame rate as video the motion appears perfectly smooth, as opposed to NTSC at 30 frames (OK, 29.97, whatever) where there's either motion judder or motion blurring except with modern equipment that can correct for them. Lee M 20:14, 4 Nov 2004 (UTC)

It should be incorrect. Most TV sets either work at 50Hz or 60Hz framerate. 24/25/29.975 refers to the frames per second (framerate), as TV broadcasts are interlaced (each redraw is done two-pass: first pass for odd lines, second redraw for even lines). RAM 02:16, 24 August 2006 (UTC)

[edit] jidder, etc.

"Ignoring color, all television systems work in essentially ": say if analog, non analog or both.

Also jidder: jitter.

[edit] System I

I thought System I was 29.975-30fps. Can anyone check it out? My mom's a TV technician, and she once told me that PAL-I is 60Hz, which almost certainly means ~30fps interlaced. (Local TV stations here uses PAL-B/G, and she once modified an old PAL-B/G TV set so I could use a PAL-I game console on it).RAM 02:09, 24 August 2006 (UTC)

• There's no bloody way PAL-I works on 60hz!! ALL the countries it has been introduced [with the exception of Macau, maybe], use 230v AC supply, at 50hz. If we used a 60hz signal, it'd bust our tv's :S (158.125.9.4 01:41, 18 November 2006 (UTC))
• PAL-I is 50 Hz interlaced (25 fps). 100% definitely. Your mom's mixing it up with PAL-M or something. The mains frequency is irrelevant. Harumphy 11:47, 18 November 2006 (UTC)

[edit] Channels

I have two questions, and they may be related....first, what does the chart mean by "b/w"...is it black and white? second, i think someone should mention the channel width for each system....

oh, and are there any multi-standard TVs that read both NTSC and PAL? (i know of SECAM/PAL TVs...) The Legendary RaccoonFox • Talk • Stalk 20:35, 16 December 2006 (UTC)

• If there are, they'd be PAL-M/N(C)/NTSC sets, surely?? NTSC is mostly only found in North and South AMerica, not in Europe, Africa, or most of Asia. If you were a tv manufacturer, you might want to consider developing a set that could be exported to Brazil/Chile/Argentina, etc. But it wouldn't be much good in Europe/Australia, etc.

Most PAL/SECAM sets can easily be transferred between European countries, as you'd expect. Many of those countries also use the same broadcast system (i.e. B/G), so they can watch broadcasts regardless of colour format. (RM21 06:02, 19 December 2006 (UTC))

[edit] Frames

This section concerns the explanation about frames. Scetpfe 16:58, 24 February 2007 (UTC)

About: "Usually it is closely related to the frequency at which the electric power grid operates, to avoid the appearance of a flicker resulting from the beat between the television screen and nearby electric lights."(end quote)

I propose that only pre-1970 sources be used for this explanation because this is the period when the field rate decisions were made world-wide, and it's also the same period where only one explanation ("Hum Bars", not the above explanation) was ever given for this choice.

The rest of my talk are thoughts on why I think the pre-1970 literature is correct, and can also serve as the skeleton of an expanded explanation if the appropriate sources are cited.

I think there is indeed a small flicker effect as mentioned in the present Wikipedia article, but I've found that in CRT-based computer monitors, a mis-match produces even less flicker. I use 72 or 75 Hz vertical refresh instead of 60 Hz because it reduces considerably the amount of flicker. That's one reason why I think the present Wikipedia explanation is not correct.

Flicker occurs from a combination of two properties, which is screen brightness and field refresh rate. Reducing one property can offset the increase in the other property. But the tradeoff is very disproportionate. Tests have shown [need to find a quote. I've read this in more than one place.] that an increase in field frequency from 50 Hz to 60 Hz requires a ten-fold reduction in pictures brightness in order to keep the same level of perceived flicker.

Yet, European TV systems use only the 50 Hz rate, causing the need to mitigate the flicker in CRT-based European TV with a combination of longer phosphor persistance, lower picture brigtness and viewer adaption over time. So why would the Europeans still use 50 Hz instead of 60 Hz in order to match the European power grid frequency of 50 Hz?

The reason is another kind of beat phenomenon even worse than the 50 Hz flicker. It's "hum bars" caused when imperfections of the TV receiver's AC to DC power conversion circuitry (power supply) leaks some of the AC into the DC (this leakage component known as power supply "ripple"). This produces the horizontal shadow-like bars, and also some geometric picture distortion near the bars. If these bars, along with the accompanying geometric picture distortion, are also allowed to move up or down, the movement calls attention to the bars, making them much more noticable.

It's too expensive to reduce the hum bars completely to zero, so an almost invisible degree of hum bar is allowed in the TV design. If the hum bar is not allowed to call attention to itself by moving, then the reciever cost can be reduced considerabley by allowing a greater intensity of hum bar before it becomes noticable.

As the electrolytic capacitors inside the TV wear out or dry up over time, the hum bar becomes more intense, and it then becomes even more important that the hum bars not be allowed to move.

And the way to stop this movement is to make the field repetition rate the same as the power grid frequency, which is the AC power going into the TV set.

After about 1985, "switch-mode" power supply design, which often completely removed hum bars, became popular, making this issue moot after 1985. But by then, the field frequencies have since long been chosen. Scetpfe 19:59, 24 February 2007 (UTC)

[edit] Modulation

This concerns "At the time television was developed, the vestigial sideband was easier to accomplish than true single-sideband modulation; with today's technology, there is no reason for it except to be compatible with existing technology" (end quote)

Actually, in the United States, digital television also uses amplitude modulation with a vestigial sideband scheme, although it's indeed much closer to true single-sideband operation than in the analog system. It uses this not for compatabiity, but because filtering in the receiver would otherwise be prohibitively expensive. Scetpfe 20:21, 24 February 2007 (UTC)

[edit] Audio

"Most commonly, the audio and video are combined at the transmitter before being presented to the antenna, ..."

I think this is more clear: "Most commonly, the audio transmitter output is combined with the video transmitter output before being presented to the antenna, ..." Scetpfe 20:26, 24 February 2007 (UTC)